The present invention relates to an image collation technology.
As a face image collation technique, a collation technique using an image of a face not looking straight ahead is available. This collating technique using an image of a face not looking straight ahead is very convenient, since it is unnecessary to force the target individual to turn their face toward an imaging device.
As an example of a face image collation technique, the following Patent Document 1 discloses that the positions of characteristic areas or points (characteristic points), such as eyes and mouth, of which brightness values change considerably on an image of an object, are detected from an image of a registered object and an input image of a target object respectively, and an image of which positions of the characteristic points are most similar with the input image is determined, whereby a position, posture or the like of the target object is calculated.
The following Non-patent Document 1 discloses that combinations of corresponding points between two images are determined, out of minor characteristics, such as moles, wrinkles, freckles and tiny bumps on the face, as the corresponding characteristic points, and a collation score of the two images is calculated based on the similarity and position of each of the corresponding characteristic points.    [Patent Document 1] Japanese Patent Document No. 4167390    [Non-patent Document 1] “Face image matching using corresponding feature points on detailed textures”, Technical Report of IEICE, Vol. 108, No. 198, PRMU 2008-75, pp. 175-180. September 2008, Yasushi Hamada, Hitoshi Imaoka
According to the technique disclosed in Patent Document 1, however, a part of the characteristic points existing on the front area of a face (e.g. eyes, mouth) cannot be detected if the target individual is looking aside, and therefore collation accuracy with a reference image drops.
In the case of the technique disclosed in Non-patent Document 1, if the imaging direction of a collation target object is quite different from the imaging direction of a reference object, corresponding characteristic points cannot be detected, or enormous processing is required to detect corresponding characteristic points.